Ancient fish fossils with preserved muscle tissue offer a glimpse at how necks evolved in early vertebrate animals. The fossils also offer a puzzle: The fish had specialized abdominal muscles found today in land animals, but not in fish, paleontologists report June 13 in Science.

The 380-million-year-old fossils come from Western Australia’s Gogo Formation and contain three-dimensional details of neck, body and tail muscles. The specimens represent several genera of predatory fish armored i...